Echoing other posters, if the itineraries are close enough for you I'd recommend doing a little thinking about what you expect out of a cruise and then learn a little about each line and ship. Go to the bookstore or library and find books that provide general information on cruising. They usually have profiles of "typical cruisers" on each line - though nobody claims to be typical
My gut says you would enjoy either cruise.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013